This invention relates to an apparatus for measuring a nuclear magnetic resonance (NMR) signal from hydrogen, phosphorus, or the like in bion and imaging the density distribution of nuclei, distribution of relaxation time, and the like, and particularly to a coil used for generating or receiving a high-frequency magnetic field.
There have been widely used the X-ray CT and ultrasonic imaging apparatus for the nondestructive inspection of an internal structure such as of a human brain and abdomen. A recent successful attempt to conduct similar inspection using nuclear magnetic resonance phenomenon has revealed the possibility of information acquisition which could not have been accomplished by the X-ray CT or ultrasonic imaging apparatus. In the inspection apparatus using nuclear magnetic resonance phenomenon, it is necessary to identify signals from an object under test separately for each portion of the object. A known method is that a gradient magnetic field is applied to an object under test to produce a different static magnetic field at each portion of the object so that positional information is acquired from a different resonance frequency or phase encode value of each portion. The fundamental principle of this method is described in Journal of Magnetic Resonance, Vol. 18 (1975), pp. 69-83, and also Physics in Medicine & Biology, Vol. 25 (1980), pp. 751-756.
Based on the fact that the SN ratio in NMR increases in proportion to the power of about 1.5 of the static magnetic field H, an attempt is being made to improve the SN ratio by producing a high-intensity magnetic field with a superconductive magnet. Conventionally, solenoids and saddle-shaped coils have been used as a coil for NMR. In this case, the resonance frequency rises as the magnetic field intensity increases, causing the self-resonant frequency of the coil to come close to or exceed the NMR frequency, resulting in a degraded sensitivity of the receiving coil or degraded efficiency of generating a high-frequency magnetic field by the transmitting coil.
A high-frequency coil overcoming this problem is described in Journal of Magnetic Resonance, Vol. 36 (1979), pp. 447-452. The coil known by the name of Alderman and Grant coil or slotted resonator has a structure as shown in FIG. 9, in which two ring-shaped inner conductors 81 and 82 are disposed coaxially, with two-piece outer conductors having wing portions being located in close vicinity to the inner conductors. The capacitive coupling between the inner and outer conductors form a loop circuit 84-82-83-81, and by application of a high-frequency current through lead wires 85 and 86 connected to the end of the conductors 84 and 81, respectively, a high-frequency magnetic field is generated in the central section in the direction shown by the arrow 88. The structure allows the detection of the high-frequency magnetic field, i.e., an NMR signal, at the section 88. Capacitors 87 are connected between the two outer conductors 83 and 84 for adjusting the resonance frequency of the coil.
The Alderman coil has the advantage of providing a higher self-resonant frequency. On the other hand, however, unbalanced capacitive coupling between the conductors is apt to create an uneven coil current, resulting in an uneven high-frequency magnetic field at transmission and an uneven sensitivity at reception.